Solid state transducer (“SST”) devices are used in a wide variety of products and applications. Some solid state transducers can emit electromagnetic radiation in the visible light spectrum, thus being useful in, for example, mobile phones, personal digital assistants (“PDAs”), digital cameras, MP3 players, and other portable electronic devices for backlighting and other purposes. SST devices are also used for signage, indoor lighting, outdoor lighting, vehicle lighting, and other types of general illumination.
FIG. 1A is a cross-sectional view of a conventional SST device 10a having lateral contacts. As shown in FIG. 1A, the SST device 10a includes a light emitting diode (“LED”) structure 30 on a growth substrate 17. The LED structure 30 has an active material 15 positioned between layers of N-type GaN 16 and P-type GaN 14. The active material 15 contains gallium nitride/indium gallium nitride (GaN/InGaN) multiple quantum wells (“MQWs”). The SST device 10a also includes a first contact 18 on the P-type GaN 14 and a second contact 19 on the N-type GaN 16. The first contact 18 typically includes a transparent and conductive material, e.g., indium tin oxide (“ITO”), to allow light to escape from the LED structure 30. In operation, electrical power is provided to the SST device 10a via the contacts 18, 19, causing the active material 15 to emit light.
FIG. 1B is a cross-sectional view of another conventional LED device 10b in which the first and second contacts 18 and 19 are opposite each other, e.g., in a vertical rather than lateral configuration. During formation of the LED device 10b, a growth substrate, similar to the growth substrate 17 shown in FIG. 1A, initially carries an LED structure 30 having an N-type GaN 16, an active material 15 and a P-type GaN 14. The LED structure 30 on the growth substrate (not shown) is attached to a carrier 21 using the first contact 18. One side of the first contact 18 is attached to the P-type GaN 14 and the other side of the first contact 18 is attached to the carrier 21. The growth substrate can be removed, allowing the second contact 19 to be disposed on the N-type GaN 16. The structure is then inverted to produce the orientation shown in FIG. 1B. In the LED device 10b, the first contact 18 typically includes a reflective and conductive material (e.g., silver or aluminum) to direct light toward the N-type GaN 16.
The SST devices are typically manufactured on wafers, e.g., semiconductor wafers. FIGS. 2A-2C illustrate a conventional technique for producing dies having the vertical configuration shown in FIG. 1B, using multiple wafers that are bonded together face-to-face. FIG. 2A illustrates a phase in the process at which an overall wafer assembly 40 includes first and second subassemblies 40a, 40b, e.g. two wafers. The first subassembly 40a includes the growth substrate 17, the active material 15 disposed between N-type GaN 16 and P-type GaN 14. Both the first and second subassemblies 40a, 40b include a first bond material 11 and second bond material 12. The first bond material 11 can be Ni and the second bond material 12 can be Sn. The first subassembly 40a is oriented such that the first and second bond materials face the first bond material 12 and the second bond material 11 of the second subassembly 40b. The first bond material 11 of the second subassembly 40b is disposed between the carrier 21 and the second bond material 12. The first materials 12 of the two subassemblies 40a, 40b are joined to form a composite wafer assembly, as explained in more detail in reference to FIG. 2B.
FIG. 2B illustrates the wafer assembly 40 having a bond material 20 formed during the process of joining subassemblies 40a, 40b. The bond material 20 has a layer of the second bond material 12 (Sn) between the layers of the first bond material 11 (Ni), e.g., an intermetallic compound (IMC). The thickness of the bond material 20 is typically in the range from 3 to 8 μm.
FIG. 2C illustrates the wafer assembly 40 after the growth substrate 17 has been removed. Additional conventional processing steps are performed after this point, including singulating SST dies from the wafer assembly 40 in preparation for packaging the SST dies.
One feature of the foregoing process is that it can result in high residual stresses in the intermetallic compounds at the bond material 20. Additionally, a wafer “bow” can be induced by the CTE mismatch between the LED structure or die material 30 and the carrier 21, thus adding more stress to the bond material 20. The thick, highly stressed bond material 20 in turn causes difficulties during die singulation. For example, FIGS. 3A-B illustrate a die singulation process using a mechanical saw 46. A wafer 60 has bond material 20 connecting the die material 30 to the carrier 21. The mechanical saw 46 singulates the die material 30 by cutting through the exposed portion of the bond material 20. Due to high stress in the bond material 20, the singulated dies can delaminate at the interface between the bond material 20 and the carrier 21. If the bond material 20 is tough, e.g., it has high ultimate yield strength, the delamination can occur inside the carrier, as illustrated in FIG. 3B. Such a delamination renders the dies 80 nonusable, thus lowering manufacturing yields. Consequently, even though die singulation using mechanical saw is advantageous because of its low cost, it may not be practical for SST wafers because of the low manufacturing yield.
One approach to address the foregoing delamination problem is to replace the mechanical saw process with a laser singulation process, which may reduce the incidence of delamination. However, the laser singulation process requires many passes of a laser beam to remove material through the entire wafer thickness (hundreds of microns) and ultimately the laser singulation process becomes very expensive. Accordingly, there remains a need for cost effective SST wafer singulation methods that do not result in excessive SST delamination.